Magmatic Carbon Dioxide Emissions From Mammoth Mountain, California -- A Decreasing Trend From 1996 to 2007

Mammoth Mountain is a Pleistocene volcano along the crest of the central Sierra Nevada Range and comprises about 20 domes of dacitic to rhyolitic composition extruded between about 110 and 57 ka. The most recent volcanic activity consisted of a few small phreatic eruptions on the north flank of the mountain about 700 yrs ago. Magmatic CO2 emissions from soils on Mammoth Mountain were first detected and measured in 1994 but later, based are 14C in tree rings, were shown to have begun in 1990. The CO2 is released during decompression and cooling of small magma bodies that intrude the crust beneath the mountain. The magmatic gas is thought to have collected in a sealed-, natural-reservoir, 1-4 km deep, which was breached by crustal deformation during an episode of seismic unrest in 1989. Recent measurements of the diffuse emission of magmatic CO2 at Mammoth Mountain show that the total output of CO2 decreased by 80% between 1996 and 2006. CO2 discharged from steam vents in fumarolic areas is minor and was not quantified. Estimates of the total mass of CO2 emitted from soils have been made based on kriging of ground-based, closed-chamber CO2 flux measurements. In five areas where measurements were made in 1996 and 2006, the rate of CO2 emission decreased by 58 to 85% in the individual areas. In the same period the total output of CO2 from the five areas decreased from 495 to 100 tonnes per day. The decrease in CO2 emission is consistent with the depletion of gas stored in a reservoir that filled over a period much longer than the 18 years since the reservoir seal was breached in 1989. Measurements of magmatic gas emissions at volcanoes such as Mammoth Mountain may not always provide reasonable estimates of the volumes of recent shallow degassing intrusions because the source of these emissions may be accumulated gas from older intrusions. Preliminary estimates from data being collected in 2007 indicate that the trend of declining CO2 emissions continues.